# Q&A（FAQ）How to choose Circuit Breaker?

Please keep the following in mind when choosing a circuit breaker to protect the input circuit of switching power supply.

## 1. Number of Poles

### in case of Single-phase electric power

There are AC(L) and AC(N) power line, and AC(N) is the earth electric potential.
If AC(L) and AC(N) can be distinctly known, a single-pole trip breaker is applicable to shut off AC(L) power line. But if AC(L) and AC(N) can't be known clearly, you have to choose a two-pole trip breaker to shut off both of them.

### In case of Triple-phase electric power

A three-pole trip breaker is necessary to shut off the whole phases.

## 2. Trip characteristics and rating

Generally, the input circuit of switching power supply is capacitor input type, therefore when power is turned on, a strong surge current, called inrush current, will occur instantaneously in the circuit. Furthermore, inrush current flows through circuit only in several milli-seconds [mS] but it becomes 10 times greater than normal input current. Circuit breaker may be shut off by the inrush current depending on its characteristics. Therefore, circuit breaker should be chosen not to be tripped by the short term inrush current. Generally, it is called "Slow-blow" type breaker.

Fig.1 Example of inrush current

Fig.2 Example of trip characteristics of breaker at 5A

The area encircled by the two curves is the operating range of circuit breaker. Suppose the time of inrush current is 2mS, according to the above graphs, it is out of the operating range of circuit breaker.

Considering the input characteristics of switching power supply, when input voltage phase is 90˚ or 270˚, a peak inrush current will occur and flow through the circuit, however, it will return to 0A in 5mS maximum.

Based on the value of inrush current listed in our catalogue, a breaker which will not operate within 5ms should be chosen. Furthermore, the values of inrush current are measured only when input voltage is set at rated input voltage mostly 100VAC and 200VAC. If you want to use a power supply with higher input voltage than rated input voltage, figure out the multiple of input voltage (100V/200V) and multiply it by the inrush current value listed in our catalogue. (Ex. If [input voltage = 220V], [multiple = 220/200 = 1.1]).

Besides the above considerations, it is also necessary to take the operating temperature into consideration to choose a breaker which will not trip even though a power supply is being used at its maximum operating range.

And finally please pay more attention to the fuse of power supply, because circuit breaker may not operate when power supply has broken down due to fuse blowing.

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